As for the curb on your English bridle... It is very confusing to the horse to direct rein with a curb bit, not to mention improper. Also, if you pulled back too tightly, you could end up over-correcting your horse, causing him to rear or throw his head. I would NOT ride with a tight rein if you do use your curb bit. Continue to neck rein on a loose rein and ride like you would when going western. The horse doesn't really care what bridle the bit is attached too .
JustDressageIt: True, a TRAINER can do whatever he/she wants with whatever bit they are comfortable using, but to advise an apparent amateur on a message board that it's okay to direct rein with a long shank curb bit is utterly irresponsible. Both horse and rider could possibly end up hurt.
If someone is on here asking if something is okay, then it's pretty safe to assume that they don't have the experience required to make that decision on their own. That means that this OP likely doesn't have the experience required to properly use a solid mouth curb bit with the level of direct contact used by most English riders.